For soft x-ray or EUV projection lithography there is a need for a high repetition-rate laser plasma source that can radiate at a specific wavelength of approximately 13 nm in the EUV part of the spectrum and capable of operating at approximately 1200 Hz. This type of source should comprise a compact high repetition-rate laser and a renewable target system capable of operating for prolonged periods of time. For a production line facility, there would need to be uninterrupted system operation for periods of approximately 3 months. That is, uninterrupted operation for some 10.sup.9 shots. To make the irradiation system cost effective, current system cost scenarios indicate the unit shot material cost must be in the vicinity of $10.sup.-6 per shot. Target sources for laser plasma soft-x-ray projection lithography have encompassed several systems such as tape driven targets and solid targets and frozen gases which all have inherent problems.
U.S. Pat. No. 5,151,928 to Hirose discloses a system which uses film type tapes as a target source. However, tape driven targets are limited mass targets that are difficult to construct, prone to breakage and produce low velocity debris that can damage the other components such as the mirrors in the laser output system. Furthermore, using and replacing the tapes is generally cumbersome and costly.
Solid targets also have many problems. Known solid target sources include rotating solid wheels of Sn or tin, or copper or gold. These sources inherently can produce various ballistic particles types that can emanate from the plasma in various directions. When using tin as the target, the interaction of the laser light with the plasma, causes highly stripped, energetic ions to be formed by collision ionization. These ions can stream ballistically from the plasma region with velocities of between 10.sup.6 and 10.sup.7 centimeters per second. Furthermore, after the interaction of the plasma and the tin target, hot clumps or clusters of target material are boiled off of the target. These material has been determined to be another form of debris particles that can have velocities in the range of 200 to 2500 centimeters per second. The sizes of these particles range from less than 1 to greater than 200 micrometers. These larger particles can cause cold particle cratering on many of the components used to construct visible and x-ray optical components such as the mirrors and are capable of puncturing thin-film x-ray filters. All of these debris size particles can degrade the surface quality or the operating performance by overcoating the optical elements in the laser output system. Likewise, other solid targets besides tin can produce similar debris type particles.
Frozen gasses such as Krypton, Xenon, and Argon have also been tried as laser source targets, and also have problems. Besides the exorbitant cost required for containment costs, these gasses are also quite expensive and would be difficult to have a continuous high repetition rate that would cost significantly greater than $10.sup.-6. Moreover, all previous embodiments of frozen gas targets have envisaged massive (continuous) large-mass pellets targets which produce particulate debris.